An electroscope is a scientific device used to detect the presence of an electric charge on a body. It was invented by British physician William Gilbert in the year 1600. An electroscope detects the charge based on the Coulomb electrostatic force which causes the motion of test charge. It can be regarded as a crude voltmeter as the electric charge of an object is equal to its capacitance. An instrument that is used to measure the charge quantitatively is known as an electrometer.

An electroscope often consists of a metal rod with a knob on the top and a pair of metal leaves connected at the bottom. This rod is inserted into a one-hole rubber stopper that is further fitted into a flask. The flask contains the rods lower part, which includes the metal leaves. In the case of the uncharged electroscope, i.e., when no charge is present, the metal leaves hang straight down. When an object with a charge is brought near an electroscope, one of two things can happen. An electroscope responds to the presence of a charge by moving electrons either into or away from the leaves. In both cases, the leaves separate. It is important to note that the electroscope cannot determine if the charged object is positive or negative – it is only responding to the presence of an electric charge.

Types of electroscopes include:

• Pith-ball Electroscope: It consists of one or two small balls made of pith, which is a lightweight, spongy material.
• Gold-leaf Electroscope: It is made up of a metal detector knob on top, which is connected to a pair of gold leaves hanging from the bottom of the connecting rod.
• Vernier Electroscope: In this electroscope, the plate, support stand, and needle are all made of a conducting material that allows both the free flow of electrons and the distribution of excess charge throughout the electroscope.

The working principle of an electroscope is based on charge induction, the atomic and internal structure of the metal elements, and the notion that unlike charges attract while charges repel each other.